98 Explorer whistling noise

I have a '98 Explorer, 5.0 v8, AWD and on the highway somewhere around 2000rpm's there is a very distinct whistling noise coming from the engine.  Slightly above or slightly below 2000 it doesn't happen.  For instance about 65mph on the highway there is nothing, but if you go up a hill or something and have to give it just a little more gas to maintain speed then the whistling starts.  I was thinking maybe it was some kind of vacuum leak or something.  Any ideas?

Also, it does have an aftermarket air intake installed.

nothing?